There are a number of visual and other functional benefits in printing only part of the surface area of a substrate. For example, it is common to partially print a substrate with one or more colors to allow the revealed substrate which is left exposed to form part of the required design.
White is the most common color of substrate to be printed over part of its area and revealed in other parts, firstly because it is easiest to achieve the desired perceived color of other colors if they are printed on white, especially if such colors are formed by transparent or translucent inks. Secondly, white forms a good contrast to many other colors and so renders graphic designs easily visible. Thirdly, white commonly forms a significantly high percentage of many designs. Fourthly, the mass processing of white substrates provides economy and efficiency in production, by standardization of the base color, if not the material specification. Fifthly, white forms the normal background to four color process printing, in which four colors (black, cyan, magenta and yellow) are typically printed in dot patterns onto a white background, the size and/or spacing of the dots of each color being varied according to "color separations" to be replicated or by digital printing techniques utilizing Raster Image Processing (RIP). From above a minimum distance, the eye cannot resolve the individual colored dots but the colored dots merge to give a combined perceived color at any position on the printed product.
Conventional printing processes typically suffer inexact registration, owing to
i) printing machine error or "tolerance" in delivering ink or other marking material, PA1 ii) the dimensional instability of a liquid ink or other marking material in liquid state on a substrate, PA1 iii) the dimensional instability of a substrate through temperature and humidity changes between printing "passes" (printing of individual layers), and PA1 iv) the error or "tolerance" in delivery of a substrate into the printing position.
For many products, this lack of registration, or lack of being able to print ink on a substrate exactly where intended, is not important. However, there are a number of products which can be very adversely affected by such lack of registration, one example being unidirectional or other vision control products, such as those disclosed in British Patent No. 2165292, which includes methods of printing with substantially exact registration and methods of overcoming the limitations of registration error of conventional printing methods. Such products typically comprise the partial printing of a transparent substrate with a fine pattern in the form of dots or lines with surrounding or intermediate transparent areas or of a grid pattern surrounding transparent areas.
A cross-section taken through such partially printed substrates will typically be in the form of a continuous substrate material on which are alternate printed portions and unprinted portions. These printed portions typically comprise superimposed layers of marking material. It is generally advantageous for a plurality of layers within some or all of the printed portions to be in substantially exact registration with coterminous boundaries. For some products, it is advantageous or essential for at least one layer to not extend over the whole area or areas of another layer but to lie within the other layer, either wholly spaced within or partly spaced within and partly having a coterminous length of boundary. When the cross-sectional dimensions of the printed portions of such a printed product are small and it is desired to superimpose more than one layer on such printed portions, the registration error of conventional printing processes can severely prejudice the achievement of the desired visual or other performance criteria. The critical factor is the registration error or tolerance of the printing process compared to the cross-sectional dimensions of the printed portions.
In the case of conventional four color process printing (sometimes referred to as four color half-tone printing) or digital four color process printing, the size of the individual dots of color are very small in relation to the area of the background substrate, which is typically white and made of paper, card or plastic materials. Substantial lack of registration in the printing of the individual dots of different colors is normally acceptable, as the individual dots of one color are not perceived as individual dots but are combined with the differently colored dots to provide the required overall impression. Lack of registration between the dots of various colors is only generally perceived as a lack of sharpness of design boundaries within the design, such as the edges of insignia seen against a background color. The observer sees what is printed. Only if the observer knows that the desired degree of edge clarity is different to that observed, or if the lack of registration is such that color "halos" are seen at color boundaries, is the lack of registration recognizable.
However, if the requirement is to print a relatively fine pattern of background color, such as white dots, then superimpose one or more single colors, of uniform hue, intensity and tone, or four color process colors, on some or all of these dots, the lack of registration of the printing process can have a significantly deleterious effect on the functional performance compared to that intended. For example, the perceived colors of an image or design will vary over the area of the substrate from the desired colors owing to the visual interaction of the unregistered layers. If a pattern of 1 mm sided square white dots are intended to be covered with 1 mm sided square dots of a different color, but there is a registration error of 0.2 mm in two orthogonal directions on plan, as in FIG. 1 of the accompanying drawings, then 36% of the desired area will appear white and have a corresponding effect of 0.36 mm.sup.2 white on the overall printed area of 1.36 mm.sup.2. If the substrate is black and the different color is formed by transparent ink, the different color will be substantially invisible against the black substrate and the 0.36 mm.sup.2 of white will be seen in combination with the 0.64 mm.sup.2 area of the different color, which will appear consequently "whitened" in this area. Such alteration from the desired perceived color will be most noticeable compared to other individual squares making up the pattern where the error in registration differs and particularly compared to any squares in which the different color substantially covers the white. The different color was intended to appear uniform over an area of panel, it will instead appear to be shaded, of differing tone.
If the substrate is transparent, such lack of registration will be typically visible from the other side of the substrate as well, the overlapping different color in the above example being visible as well as the white square, which will typically not be desired.
There is another problem, that undesirable perception of color can be caused by lack of opacity of individual ink layers. In the above example, if the white and different color were printed on a transparent substrate, when the white is observed from the other side of the substrate, this could be modified by the different color, which could be exacerbated by the illumination condition behind the substrate.
From the printed side of the panel, the different color covering the white area would be perceived as being a whitened or a lighter color tone of the different color. It is common in printing to overcome such lack of opacity by printing more than one layer of a color, to achieve the desired or necessary degree of opacity. However, if the registration error is relatively large compared to the cross-sectional dimensions of the printed portions being printed, the lack of registration will result in yet further areas of different perceived color where the edges of the desired shape overlap through lack of registration, as well as poor edge definition.
Methods of substantially exact registration printing utilizing differential adhesion of a layer of marking material onto a substrate are disclosed in British patent numbers 2118096 ('096), 2165292 ('292) and 2188873 ('873). The first layer of one material defines a print pattern (referred to as a silhouette pattern in '292). This first layer may be in the form of the print pattern or be a stencil of the print pattern. A second layer of a different material, typically a printing ink, is applied over the print pattern and beyond the boundaries of the print pattern. This second layer adheres within the print pattern but does not adhere to the substrate outside the print pattern. The second and any further layers typically comprise printed ink, which is cured and subsequently removed from outside the print pattern, for example by the application and removal of self-adhesive tape or by high pressure water hosing. The curing regimes of the ink layers to enable removal of unwanted ink, and the means of such ink removal are difficult, costly and time consuming.
The prior art also includes Japanese Unexamined Patent Publication number 33723/78 to Kawai entitled "Method of Thermally Transferring Metal Foil onto Outer Surface of Hard Substrate". This document also outlines the previous prior art of hot foil stamping and the hot roller transfer from a carrier of a printed design onto a substrate. Kawai discloses the method of thermally transferring a continuous foil of metal or synthetic resin onto a thermo plastic synthetic resin ink, paint or glue which is first adhered to and cured on the outer surface of a plate like or molded hard substrate such as glass, ceramic, metal, marble, cured synthetic resin or the like. The method of Kawai does not allow for the partial coverage of the first layer which is adhered to the substrate.
Another problem when making products according to the '292 invention by the prior art methods is that Moire fringe patterns can result from using four color separations superimposed on a dot or line "silhouette pattern".